The 'surge' of corticosteroid in fetal plasma during late gestation has been implicated in the initiation of parturition and the maturation of enzyme systems in organs such as the lung, liver, adrenal medulla and thyroid. But in all species studied, the mechanism responsible for increased secretion in the fetus remains unclear. The hormone adrenocorticotropin (ACTH) is well established as the primary regulator of adrenocortical cellular growth and secretory function during fetal and adult life. However, no increase in fetal plasma ACTH has been observed before the corticosteroid surge in sheep or humans, although differential responsiveness of the fetal adrenal cortex to ACTH at various gestational ages and a possible role of extra-adrenal inhibitory factors have been proposed. The possible steroidogenic effect of alpha-melanocyte-stimulating hormone (alpha MSH) on fetal adrenocortical function is controversial, and we could not demonstrate any such action in fetal lamb or rabbit. Prolactin and growth hormone (GH) potentiate the steroidogenic effect of ACTH in adult rats but prolactin had no such effect in fetal lamb. We have investigated the steroidogenic properties of GH, and report here that it stimulates adrenal steroidogenesis in the fetal but not maternal rabbit in vitro and in fetal but not maternal sheep in vitro as well as in vivo.